Holography is known as a two-step process: recording and reconstruction. In the recording step a hologram is recorded, i.e. interference between two waves, with a mutual degree of coherence in space and time. These beams are the “object beam” (arriving from the object toward the recording material) and the “reference beam” (directly illuminating the recording material). After the recording step the recording material is usually processed (chemically or otherwise) in a manner that creates the hologram. In the reconstruction step a reconstruction of the hologram is performed, usually by illuminating the hologram with a beam equivalent to the reference beam or the object beam. Employing this method of illuminating the hologram, allows viewing the ‘source’ of the other beam, i.e. the image of the object (which can be a “virtual”, or a “real” object), or the image of the reference beam (usually a point source).
Holography was developed as an analog device, operating in many ways like a lens. As digital methods have been developed, the scientific community started to search for possible ways to create a hologram and also reconstruct it digitally. The reconstruction process was the first one to be performed, since it is simpler and also independent of its recording.
By performing an interference between a spherical wave, emerging from a point source (i.e. through a pinhole), and a plane wave—a basic hologram is produced. It is possible to produce these two waves in a coaxial manner, or at a certain angle between them. Since both waves originate from the same radiating source (with a certain spatial and temporal degree of coherence), it is possible to achieve a good mutual coherence between them, if their paths are within the coherence length of the radiating source. If reconstruction is performed by a plane wave, similar to the one applied during the recording process, one obtains a virtual image of the point source, that acts as a diverging (negative) lens and, at the same time, a real image for a convergent one (positive lens).
The focal length of this lens and its area are functions of the geometrical recording conditions and also the recording and reconstructing frequencies (i.e. wavelengths). Therefore they are spatially and temporally frequency selective. Holography was developed for various applications, among them for reconstructions with white light and in real-time.
Holography is also used for collecting solar energy, where a concentrating holographic lens is used. The holographic lens is provided with a multiple holographic patterns each corresponding to a specific elevation and location of the sun, so that as it travels in the sky different patterns go into action each one corresponding to a particular solar location (see U.S. Pat. No. 4,490,981 (Meckler)).
Real-time reconstruction holography has the advantage of observing variations of the object (static and dynamic movements of its surface, changes in the index of refraction in the material under test, etc.). Certain photosensitive materials change their thickness or indices of refraction (commonly named ‘phase materials’) instead of their opacity. To these materials belong also films of dichromatic gelatin and photoconductor-thermoplastic films. Using these materials, it is possible to record a hologram at one time and erase it at some other time, and then make another hologram [Collier R. J., Burckhardt C. B., and Lin L. H., “Optical Holography”, Academic Press, (1971)]. Approaches towards an electrically controllable hologram were performed in the past. Liquid crystal (LC), which was produced at different heights, and excited by electrical means is one example [Slinger C., et. al., “Electrically controllable multiple, active, computer-generated hologram”, Optics Letters, 22, (14) 1113–1115, (15 Jul., 1997)]. Another example is adaptive computer generated hologram in LC, using hybrid feedback systems and interpolation method [Yoshikawa N., et. al., “Adaptive computer-generated hologram with hybrid feedback system”, 1966 International Topical Meeting on Optical Computing, Technical Digest, Vol. 1, 224–5]. Another example is a switchable Fresnel lens [Yun-Hsing Fan, Hogwen Ren, and Shin-Tson Wu, “Switchable Fresnel lens using polymer-stabilized liquid crystal”, Optics Express, 11, (23), 3080–3086, (17 Nov. 2003)]. However, until now a process for continuously recording and reconstructing a hologram in ‘real-time’ has not been achieved.
The present invention aims at providing a holographic lens, capable of performing processes by digital methods, which simulate in real-time the recording and reconstruction of a conventional holographic lens.
Another object of the present invention is the provision of a dynamically controllable holographic lens that changes its viewing direction in real-time, while maintaining the spatial positions of the spectral foci. This device is based on simulating the intensity (fringes) obtained at every point in the holographic plane from the incident beams, by means of computer generated processing. Thus changing, by electrical means and in real time, a property (like index of refraction) of a photo-refractive material, and therefore obtaining a Real-Time Computer Generated Hologram (RTCGH). This lens is of a type named hereinafter Real-Time Computer Generated Holographic Lens (RTCGHL). Such a lens can be applied, for example, for concentration of sunlight, as it travels in the sky to a fixed point, in order to transform it to electrical energy.
Another object of the present invention is the provision of a device that tilts the refracted beam in real-time, named hereinafter Real-Time Computer Generated Holographic Tilt (RTCGHT).
Yet additional object of the present invention is the provision of a device that splits the incident beam in real-time into several diffracted beams, named hereinafter Real-Time Computer Generated Holographic Splitter (RTCGHS).
Other objects and advantages of the present invention will become apparent after reading the present specification and reviewing the accompanying drawings.